The present invention relates to a video recording system comprising a solid state image sensing device and a video tape recording device, and more particularly to a video recording system for recording a video signal having a high resolution such as more than 1,000 horizontal scanning lines per frame on a magnetic tape.
In a solid state image sensing device for deriving a video signal of a high resolution it is necessary to increase the number of picture elements and therefore, the number of image sensing cells is naturally increased. It should be noted that an image sensing cell means a unit of a photoelectric converting element for producing a photoelectrically converted signal representing brightness information of a picture element. As is well known in the semiconductor field, there have been developed various types of solid state image sensing devices such as a charge transfer device including a charge coupled device (CCD) and a bucket brigade device (BBD), in which the brightness information is read out by transferring a mass of charges generated in response to impinging light, and an X-Y photodiode matrix device in which each cell is coupled with an analog gate formed by a MOS transistor with the brightness information being read out by deriving via the analog gate a mass of charges generated in a relevant cell in response to incident light.
In the above mentioned solid state image sensing device, the charges stored in each cell is discharged and a video signal representing the brightness information is derived by detecting a magnitude of a discharge current. Therefore, it requires a relatively long time for deriving the video signal from each cell. For instance, if the read-out time for a single cell is one micro second and the image sensing device comprises 1,000.times.1,000 cells, it takes 1 .mu.s.times.10.sup.6 =1 second for deriving the video signal representing a single frame. That is to say, there is produced a time difference of one second between a timing at which a first picture element is read out and a timing at which a last picture element is read out.
In the solid state image sensing device, an amount of stored charges is substantially proportional to a time during which light is made incident and thus, a very large difference in sensitivity is produced between respective cells and the quality of a reproduced image might be damaged to a great extent. In an extreme case, the charge might be saturated during the read-out operation and a correct video signal may not be obtained. In order to obviate such a drawback, it has been proposed to provide a mechanical shutter to shield an irradiation of the image sensing device during the read-out period. That is to say, an exposure time is controlled in accordance with an amount of stored charges in respective cells in such a manner that the charges do not saturate.
In a video tape recording device for recording a video signal derived from a video camera comprising the solid state image sensing device and a mechanical shutter, it is required that a magnetic tape travel continuously at a given constant speed, otherwise a desired scene could not be recorded accurately, because the shutter is manually actuated at random timings. Therefore, the video signal is recorded on a very small part of the magnetic tape and the magnetic tape is not utilized efficiently and there remain a large amount of tape portions on which no video signal is recorded.